The present invention relates in general to integrated circuits, and more particularly to the protection of transistors that control signals whose voltage exceeds the transistors' breakdown potential.
Wireless communications systems are presently using digital techniques to increase functionality and noise immunity while reducing cost. For example, cellular telephones and pagers receive radio frequency carrier signals modulated with digital data. A digital signal processor (DSP) receives the digital data from a demodulator and executes a preprogrammed software algorithm to convert the data to audio and display signals.
Portable wireless communications devices typically operate from a low voltage battery, e.g., 3.0 volts. However, communication between integrated circuits often is based on 5.0 volt logic swings in order to improve noise immunity for data transfers. Moreover, the software programs to operate the DSP are typically stored in a nonvolatile memory circuit which uses programming signals as high as 18.0 volts to program the memory cells. The higher voltages typically are generated with charge pumps.
Such integrated circuits are fabricated on a low voltage transistor process because the resulting die is smaller and consumes less power for a given operating speed. However, the low voltage transistors are susceptible to breakdown when controlling the higher voltage signals.
Many prior art systems control the high voltages by fabricating integrated circuits on a high voltage transistor process whose devices have breakdowns exceeding the high voltage signals being controlled. However, such integrated circuits have larger die areas and lower speeds than circuits built on a low voltage process. Other prior art circuits add process steps to produce different gate oxide thicknesses to fabricate both low and high voltage transistors on the same die. However, the increase in processing steps adds to the cost of the integrated circuit, especially because high voltage devices are needed in relatively few portions of the circuit.
Still other prior art circuits use cascoded drivers to increase the voltage handling capability of high voltage stages implemented with low voltage transistors. A cascoded driver comprises two transistors with serial drain to source connections such that both transistors pass the full load current. The cascode configuration results in only a portion of the high voltage being applied to each transistor, but each transistor must be large enough to handle the full load current. For drivers supplying high load currents, two large transistors are needed to implement the cascode configuration, which increases die area and cost.
Hence, there is a need for an improved circuit and method of controlling high voltage signals with small, low voltage transistors, thereby reducing die size and cost and improving low voltage performance.